The present invention relates to a force-transmitting drive cap for use between the hammer of a pile driver and the end of a pile being driven.
A pile driver of the non-vibratory type usually has a hammer which is reciprocated in a normally vertical direction so as to strike the upper end of a pile each time it descends, thereby driving this pile into the ground or other substrate. Normally the hammer, which may be a simple ram that is lifted hydraulically and then dropped in order to reciprocate, does not strike directly against the upper end of the pile, but strikes against a drive cap sitting on the upper end of the pile. The drive cap is of relatively hard material, normally hardened steel, whereas the pile is of a softer material, normally mild steel or even wood. Thus smooth force transmission takes place through this drive cap to the pile so that the hardened steel hammer of the pile driver does not mushroom the upper end of the pile.
It is known to provide a shock-absorbing bumper on or in the drive cap so that the enormous forces that are applied virtually instantaneously to the drive cap are transmitted to the pile over a longer period of time, thereby preventing deformation of the pile as the drive force peaks. In the simplest arrangements such a force-transmitting and force-absorbing cushion is a simple block of wood or elastomeric material that is used until it is itself destroyed, whereupon it is replaced by another such cushion or bumper.
In a known system a hydraulic or mixed hydraulic/pneumatic bumper is provided in the drive cap. In such systems the upper side of the drive cap, that is the side engaged by the pile-driver hammer, is separate from the lower side, that is the side resting on the upper end of the piston, and a compressed gas cushion is provided between them. The hammer strikes the upper side with a force sufficient to compress the gas cushion, but not with a force exceeding the upwardly effective force of the cushion on the upper side. Thus the pressurization of the chamber in the drive cap sets a limit to the force with which the hammer can strike this drive cap. Such devices cannot be used for the driving of piles in very hard ground as it is impossible to apply enough force through such a drive cap to the pile without destroying the drive cap or mushrooming the upper end of the pile. When extreme force is needed it is necessary to use the above-mentioned bumpers of wood, asbestos, or a hard synthetic resin.
Another disadvantage of the known cushioned drive caps is that they cannot be adjusted for use in driving piles in grounds having different characteristics or resistance to penetration of the pile.